First of all we need to establish that autoflowers will flower under any light cycle and you most definitely can grow them under 24 hours of light and even on a 12/12 schedule, but there are more aspects to look over than just if they will grow!
Light cycle can change autoflower plants growth, the end yield, bud density, branch formation and even plants height so growers should definitely know why plants in certain light cycles do best and in what stages of plants growth what cycles would be best!
Cannabis is a plant that requires no dark period and that is why it is categorized as a C3 plant which means that it gathers CO2 during the light period and uses it at the same time but it doesn’t gather the CO2 in the dark period like the C4 plants. This means that cannabis plants thrive if they get continuous light through their growth stages so non stop 24/0 light cycle should be better than 16/8 or 18/6. In the vegetative growth that happens in first few weeks of autoflowering plants growth you can increase the growth by up to 30 % if you leave the lights on all the time, but in later growth stages this additional growth is not so evident and you won’t get 30 % higher yield but an increase growth will be visible. This additional light is beneficial for autoflower plant but it will cost more:
For Example if a grower has a 400 W HPS lamp and the electricity price is 10 cents for KWh. In this scenario if the grower would run his HPS 80 days non stop then he would be burning 768 kilowatts ((24 h * 80 days * 400 W)/1000) that would cost him $76.80 but if he would use a 18/6 light schedule he would be burning just 576 kilowatts that cost $57.8 and he would save $19 just with this one HPS lamp. The non stop light would also decrease the lamps life span that could also be a potential saving.
After seeing these number every grower can do the math and see how much more money he/she would burn for his setup, but everyone should know that depending on the autoflower strain that dark period can decrease the plants growth by about 20 % and here you can do the real equations on how much expected yield would you lose if the autoflower plant would get 20 % less energy to use and create buds!
After doing this little experimental calculations we can see that full 24/0 light schedule is way more beneficial for autoflowers because they will grow bigger, faster and you will ultimately get more yield and the electricity costs aren’t that big compared to that increased yield. So I would suggest you to run your lights non stop and ultimately you will get more for your investment in energy as probably the plant is much much more valuable than those few dollars that you spend on electricity.
Light cycle changes can also decrease the overall heat signature of your grow and if you have any heat problems you can slightly reduce them by using a light schedule where the grow room could cool off a bit in the dark period. In this scenario there are two potential pitfalls.The first is that usually your grow area should be well isolated and continuous light should not be a problem, and if a continuous light can overheat your grow room you should definitely think about that grow room design and tweak it till heat is not a problem anymore!
The second thing that’s wrong with the option of turning lights off for cooling the grow room is that autoflowers don’t like a temperature difference that’s more than 10 degrees and if your daytime temperatures are reaching 30 C but in night your temperature drops below 20 C you can severely stress your little plant and stun it.
So you can use light cycles for grow room cooling if you have no other choices but for the most cases I would not recommend it.
In nature autoflowering cannabis plants get different light exposure cycles and the most stretching usually happens in the night time. In the night cannabis plant tries to find more light and it stretches toward the sky to do that. This doesn’t mean that in the light cannabis growth stops, this just implicates that in the night cannabis stretches more. Even if the stretching is stronger in night, the overall growth is much faster in daylight as cannabis produces energy only when it gets light shining on those green leafs.
So growers could use light cycle changes to aid bigger plants by letting them stretch out in the dark period ultimately getting bigger plants but consistent dark period will not necessarily yield the most amount of bud. I would say that if you really want to stretch your plant then a dark period should be mixed with continuous light periods where the autoflower plant could stretch at the night and get a long light schedule for more vigorous growth by branching out, making more leafs and denser buds. But if cannabis produces energy only in the light then logically the best conditions for it is continuous light throughout its life to produce the most amount of energy to grow leaves, stems and ultimately big buds.
Contrary to popular belief you should set the light schedule for more light if you want smaller plants, but smaller doesn’t mean that they would produce less, because they would have denser buds more closer branches and more total mass and they won’t be stretched.
Bud formation under 24 and 16
Autoflowering cannabis plants will produce denser buds if the light period is continuous as the plant produce all its energy from photosynthesis and more energy means more mass and denser buds. The dark period between the light cycles can increase stretching that will lead to fluffier buds. This increased stretching can only be beneficial if you have a very humid environment where dense buds could easily sustain mold or fungus but that usually can be treated with decreasing the humidity levels!
A truly stable autoflowering seeds that are 100% autoflowers will definitely yield more if it is under continuous light cycle but not all seeds are perfect and many first, second and some more generation unstable crossbred seeds will not completely flower under 24 hours of light and will produce small yields. In this scenario the dark period between light is a good deal as you’re not so perfect autoflower will get some dark period and will produce good buds.
Light cycle at the end of the plants life cycle
many growers say that a dark period increases the resin gland production on cannabis plants and that plants that are deprived of light for the last couple of days will give you stickier and whiter buds but this theory has not been proven scientifically and as much as I can tell it really doesn’t matter if you give them a light shock in form of two day dark period or not. They will give you the same amount of buds. If we think about this myth then we can boil it down to the truth, which says that cannabis plant produce energy while they are exposed to light. This should mean that you will get bigger and denser buds if you supply more light. So for autoflowers if you give them 48 hours of light oppose the 48 hours of darkness at the end of their life cycle they should produce more from that extra energy!
The only real world solution for the dark period at the end of the plants life cycle is in the situation when you need to harvest prematurely and and you can’t wait for the real harvest. In this situation giving autoflower plant a complete light deprivation stress may trigger it to form denser buds as it thinks its life is coming to an end so it produces more resin to protect its seeds!
One more thing to remember is to give your plants a stable light schedule and don’t change it during its growth so your autoflowers don’t get confused and can grow to their full potential. It Is undoubtedly true that automatic plants will flower under any light schedule but cycle changes can reduce the yield and induce stress in those plants. If you have some light leaks or unplanned electricity outages then autoflowers are much more tolerant to those occurrences than regular photo sensitive cannabis and they won’t convert back to vegetative growth as it can happen with regular weed.
Autoflower light cycles outdoors
For outside growers light cycles aren’t a big deal because you can’t change the period when the sun shines and when it doesn’t so the only thing you can do is calculate the best possible time to plant your weed. Say your autoflower ganja plant will automatically flower after 25 days and it will be ready to harvest in 70 days from seed then you can divide this time and plant your cannabis plant outdoors 35 days before the summer solstice as then it will get the most amount of light as nights are very short when the solstice is coming!
In case you need more help growing your plants you can check out the autoflower grow guide where I give a complete explanation of how to grow your autos!